1. Field of the Invention
The present invention relates to an electroluminescence element and a process for fabricating the same. The electroluminescence element (hereinafter also referred to as "EL element") can be used, for example, as a two dimensional light source for backlighting a meter in a vehicle, or the like.
2. Description of the Related Art
EL elements utilize luminescence of a fluorescent material such as zinc sulfide (ZnS) when an electric field is applied, and have attracted attention as an element for composing a flat panel display.
FIG. 1 shows a typical cross-sectional structure of an EL element, in which the EL element 9 comprises a glass substrate 1 of an insulating material, on which a first electrode 2 of optically transparent ITO, a first insulating layer 3 of, e.g., tantalum pentoxide (Ta.sub.2 O.sub.5), a luminescent layer 4, a second insulating layer 5 and a second electrode 6 are stacked in this order.
The ITO (indium tin oxide) layer is an electrically conductive transparent layer of indium oxide (In.sub.2 O.sub.3) doped with tin (Sn) and is widely used as a transparent electrode due to its low electric resistance.
The luminescent layer 4 is, for example, of zinc sulfide as a matrix in which manganese (Mn) or terbium (Tb) is added as a luminescent center.
The luminescent color of an EL element is determined by the kind of the additive in the zinc sulfide. For example, if manganese is added as the luminescent center, the luminescent color of the EL element is yellowish orange and if terbium is added, the luminescent color is green.
EL elements having the above structure, in which samarium (Sm) is added as the luminescent center to zinc sulfide (ZnS) to obtain red luminescence or in which thulium (Tm) is added to zinc sulfide to obtain blue luminescence, have been investigated.
Usually, the electroluminescence from a rare earth element is due to electron transition between the levels of 4f.sup.n configuration. For example, the red luminescence obtained in zinc sulfide doped with samarium is due to electron transitions for .sup.4 G.sub.5/2 level to 6H.sub.7/2 level (610 nm luminescence) and from .sup.4 G.sub.5/2 level to 6H.sub.9/2 level (655 nm luminescence).
The electron transitions between these levels are forbidden as electric dipole transitions by the parity rule, and its transition probability is low. However, for a rare earth element placed in a crystal such as zinc sulfide, the crystal field interacts with the wave function of the 4f electrons, resulting in the increase in the 4f--4f transition probability. Nevertheless, since the transitions between the 4f-inner-orbit levels of the rare earth element are originally forbidden and therefore the obtainable transition probabilities of the transitions are not high, a high luminance has not been obtained in an EL element comprising a luminescent layer of zinc sulfide with a rare earth element added thereto.
It is also known that if a halogen element such as fluorine or chlorine is further added to a luminescent layer doped with a rare earth element as a luminescent center, the luminance is improved. In this case, source materials used for such a luminescent center are, for example, samarium trichloride (SmCl.sub.3), thulium trifluoride (TmF.sub.3), or the like. Nevertheless, even if such a rare earth element halide is added to a luminescent layer, the EL element has a very low luminance, at most 1000 cd/m.sup.2 (driven at 5 kHz) for red luminescence and 10 cd/m.sup.2 (driven at 5 kHz) for blue luminescence, which is not practically suitable to be used as a display such as an EL panel.